There has long been recognized the need for inserting contraction joints into a wide variety of paved structures in order to prevent buckling and to ensure proper concrete consolidation. Such joints are crucial in providing stability to the paved structures, yet must also be constructed in such a manner as to undergo minimal deterioration when exposed to the elements. This is particularly true in the case of joints designed for use in concrete linings of water conveyance systems such as canals or reservoirs which are constantly underwater. Joints used in such systems must be able to maintain flexibility to allow expansion and contraction of the concrete lining, durable enough to withstand pressure and the elements, and engineered precisely enough to form a homogenous water-tight contraction joint in the paved lining.
In the past, various devices have been used for the insertion of joints in concrete linings of water convenience systems and other structures. These attempts generally have suffered from various drawbacks such as rapid deterioration, poor bonding capabilities, or inconvenience and expense in their application. An example of one such prior art device is the preformed mastic strip which has been used to form joints in concrete linings but suffers from all of the aforementioned problems. Other prior art devices in this field which also encounter similar problems include PVC T-strips, which are presently used in water conveyance construction.
Other devices known in the prior art include joints which are placed on the subgrade before concrete is poured over them, and these joints allow the formation of cracks in the pavement along a plane of weakness above the joint when the concrete undergoes contraction. This controlled cracking relieves the stresses in the concrete layer and thus acts to prevent further cracking or buckling. Examples of this type of joint are disclosed in U.S. Pat. Nos. 2,806,414 (Woodman) and 3,583,120 (Wangerow). In the Woodman patent, a metallic forming strip is disclosed having an inverted V-shaped central projection, two V-shaped ridges on the sides of the central projection, and a series of apertures in the strip to allow spikes to be inserted to anchor the strip in the subgrade. In the Wangerow patent, the joint structure disclosed at FIGS. 1 and 2 has a thin central fin disposed between two rail-shaped anchoring members. However, the devices disclosed in these two patents both suffer from the drawback that their thin central fins are not rugged enough to withstand the pressures associated in a water conveyance system, and, as can be observed in FIG. 2 in both patents, the fins extend only through a small portion of the overlying concrete. This distance is described in the Wangerow patent as approximately one-third of the concrete layer (see column 2, lines 54-58). As a result of the height of the fin, a substantial portion of the pavement above the fin undergoes cracking when the concrete contracts or expands. Since there is often some amount of breakup in the concrete at the interface of adjoining panels caused by movement of the panels, these cracks increase the possibility of damage and deterioration of the concrete layer and of the joint structure. This again would particularly be a problem with regard to use of these two joints in concrete-lined water conveyance systems.
It is thus highly desirable to develop a device for forming a water-tight contraction joint in concrete linings such as would be used in a water conveyance system, one which can ensure proper consolidation and prevent buckling, yet which is designed to reduce concrete deterioration and better withstand the elements. It is also desirable that such a device can be constructed simply and inexpensively, and be easily used in monolithic methods of preparing concrete linings and other paved structures.